Chiral Modification of the Pd(111) Surface By Small Organic Molecules

Chiral modification of a metal surface is one of the most successful approaches of achieving enantioselective catalysis in heterogeneous phase. In this approach, an active metal surface is directly modified by adsorption of a chiral molecule (chiral modifier), the metal is responsible for catalytic activity while the adsorbed modifier controls the stereochemistry of adsorption and subsequent reactions through substrate interactions. So far, there are three types of widely recognized metal/modifier catalytic systems: tartaric acid modified Ni catalysts for the hydrogenation of β-ketoesters, cinchona modified Pt catalysts for hydrogenation of α-ketoesters and Pd catalysts modified with cinchona for selective activated alkene where high activity and enantiomeric excess is gained in comparison to the unmodified surfaces. However, the exact manner in which chirality is bestowed to a metal surface and how that affects a chiral reaction is not well understood and warrants the development of model studies (surface science analysis on well-defined single crystal surfaces under ultrahigh vacuum conditions). These model studies and surface science analysis are necessary to promote the fundamental understanding and to facilitate the rational design of a suitable metal/modifier system which is the principal focus of this dissertation. This dissertation is primarily focused on two aspects. First, a number of complementary surface science studies have been performed to characterize four different chiral modifiers: D-alanine, (S,S) tartaric acid, L-aspartic acid and α-(1-naphthyl) ethylamine on a Pd(111) surface to gain insight into the way in which they impart chirality to the surface. Second, the enantioselectivity of the chirally modified surfaces has been measured in ultrahigh vacuum. This has been achieved by exposing the modified surface to both enantiomers of another chiral molecule (the probe), to see if there is any enantiospecific interaction between the modifier and the probe. The enantioselectivity is measured from the enantioselectivity ratio which is the ratio of relative coverages of two enantiomers of the probe on a surface modified with a single enantiomer of the modifier. Combined experimental results and theoretical density functional theory calculations suggest that the amino acid modifiers impart chirality to the Pd(111) surface by an ensemble mechanism where they work collectively to form discrete chiral templates which interact with the chiral probe propylene oxide and glycidol enantiospecifically whereas, tartaric acid and naphthylethylamine provide individual chiral motifs which interact with the probes in a one to one fashion

Local and Extended Structures of d‑(−)-Tartaric Acid on Pd(111)

The structures of d-(−)-tartaric acid on the Pd(111) surface are studied by scanning tunneling microscopy (STM) supplemented by density functional theory (DFT) calculations as a function of coverage and sample temperature. When low coverages of d-(−)-tartaric acid were dosed with the sample at 300 K, and then cooled to 120 K for imaging, doubly dehydrogenated tartrate species (C₂H₄O₆^2–) are observed, while at higher coverages, singly deprotonated bitartrate species (C₂H₅O₆^1–) are prevalent. STM images show that the tartrate species are isolated on the surface, while the bitartrate species oligomerize by extensive hydrogen-bonding interactions. DFT calculations show that the adsorption of tartrate species locally disrupts the Pd lattice thereby imparting strain to the surface. The interaction between the two tartrate species in a dimeric pair is suggested to be the contribution of three factors: the adsorption-induced stress to the surface, Coulombic repulsion between the tartrate species, and the intermolecular hydrogen-bonding interactions. Occasionally, two different kinds of kinetically controlled ordered structures are observed after long times. The first consists of intermolecularly hydrogen-bonded bitartrate species and the second comprises tartrate species interacting through the substrate. DFT calculations suggest that the ordered tartrate domains form as a result of minimizing the local strain energy of the surface and hydrogen-bonding interactions

An Infrared Spectroscopic and Temperature-Programmed Desorption Study of 1,1-Difluoroethylene on Clean and Hydrogen-Covered Pd(111)

The surface chemistry of 1,1-difluoroethylene was studied on clean and hydrogen-covered Pd(111) using a combination of temperature-programmed desorption and reflection absorption infrared spectroscopy (RAIRS) to explore whether the larger infrared absorbance of 1,1-difluoroethylene than ethylene may be used to examine reactions under realistic catalytic conditions using RAIRS. It was found that the chemistry of 1,1-difluoroethylene on Pd(111) surfaces is similar to that of ethylene with bonding occurring in both the π- and di-σ-forms. However, due to the presence of C–F bonds in the molecule, the infrared absorbances for 1,1-difluoroethylene were much larger than those for ethylene. This provides the potential for using RAIRS for in situ studies of catalytic reactions that involve alkenes

Formation of Chiral Self-Assembled Structures of Amino Acids on Transition-Metal Surfaces: Alanine on Pd(111)

The structure and self-assembly of alanine on Pd(111) is explored using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection–absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM), and supplemented by density functional theory (DFT) calculations to explore the stability of the proposed surface structures formed by adsorbing alanine on Pd(111) and to simulate the STM images. Both zwitterionic and anionic species are detected using RAIRS and XPS, while DFT calculations indicate that isolated anionic alanine is significantly more stable than the zwitterion. This observation is rationalized by observing dimeric species when alanine is dosed at ∼270 K and then cooled to trap metastable surface structures. The dimers form due to an interaction between the carboxylate group of anionic alanine with the NH₃⁺ group of the zwitterion. Adsorbing alanine at 290 K results in the formation of dimer rows and tetramers resulting in only short-range order, consistent with the lack of additional diffraction spots in LEED. The stability of various structures is explored using DFT, and the simulated STM images are compared with experiment. This enables the dimer rows to be assigned to the assembly of anionic-zwitterionic dimers and the tetramer to the assembly of two dimers in which three of the alanine molecules undergo a concerted rotation by 30°

Water-promoted interfacial pathways in methane oxidation to methanol on a CeO2-Cu2O catalyst

Highly selective oxidation of methane to methanol has long been challenging in catalysis. Here, we reveal key steps for the pro­motion of this reaction by water when tuning the selectivity of a well-defined CeO2/Cu2O/Cu(111) catalyst from carbon monoxide and carbon dioxide to methanol under a reaction environment with methane, oxygen, and water. Ambient-pressure x-ray photoelectron spectroscopy showed that water added to methane and oxygen led to surface methoxy groups and accelerated methanol production. These results were consistent with density functional theory calculations and kinetic Monte Carlo simulations, which showed that water preferentially dissociates over the active cerium ions at the CeO2–Cu2O/Cu(111) interface. The adsorbed hydroxyl species blocked O-O bond cleavage that would dehydrogenate methoxy groups to carbon monoxide and carbon dioxide, and it directly converted this species to methanol, while oxygen reoxidized the reduced surface. Water adsorption also displaced the produced methanol into the gas phase

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